Fast laser power control with improved reliability for surface inspection

ABSTRACT

An inspection system may include, but is not limited to: an illumination subsystem for directing light to an inspection specimen comprising: a power attenuator subsystem configured for altering the power level of a light beam emitted by the illumination subsystem; and a power attenuation control subsystem configured to provide control signals to the power attenuator subsystem according to a detected level of light scattering by the inspection specimen upon illumination by the illumination subsystem. A method for scatterometry inspection may include, but is not limited to: directing light having a power level to an inspection specimen from a light source; detecting light scattered from the specimen; and modifying a power level of one or more intermediate light beams within the light source according to a level of light scattering by the specimen upon illumination by the light source.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims the benefit of theearliest available effective filing date(s) from the following listedapplication(s) (the “Related Applications”) (e.g., claims earliestavailable priority dates for applications other than provisional patentapplications or claims benefits under 35 USC §119(e) for provisionalpatent applications, for any and all parent, grandparent,great-grandparent, etc. applications of the Related Application(s)). Allsubject matter of the Related Applications and of any and all parent,grandparent, great-grandparent, etc. applications of the RelatedApplications is incorporated herein by reference to the extent suchsubject matter is not inconsistent herewith.

RELATED APPLICATIONS

The present application constitutes a continuation-in-part of U.S.Provisional Patent Application Ser. No. 61/227,705, naming Wolters etal. as inventors, filed Jul. 22, 2009, which is currently co-pending, oris an application of which a currently co-pending application isentitled to the benefit of the filing date.

BACKGROUND

Fabricating semiconductor devices, such as logic, memory and otherintegrated circuit devices typically includes processing a specimen suchas a semiconductor wafer using a number of semiconductor fabricationprocesses to form various features and multiple levels of thesemiconductor devices. For example, lithography is a semiconductorfabrication process that typically involves transferring a pattern to aresist arranged on a semiconductor wafer. Additional examples ofsemiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing, etch, deposition, and ion implantation.Multiple semiconductor devices may be fabricated in an arrangement on asemiconductor wafer and then separated into individual semiconductordevices.

Inspection processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers. As the dimensions ofsemiconductor devices decrease, inspection becomes even more importantto the successful manufacture of acceptable semiconductor devices. Forinstance, detecting defects of decreasing size has become increasinglynecessary, since even relatively small defects may cause unwantedaberrations in the semiconductor device, and in some cases, may causethe device to fail.

Many different types of inspection tools have been developed for theinspection of semiconductor wafers, including optical and E-beamsystems. Optical inspection tools may be generally characterized intodark-field and bright-field inspection systems. Dark-field systems aretypically known for having a relatively high detection range. Forinstance, dark-field systems detect the amount of light that isscattered from the surface of a specimen when an incident beam issupplied to the specimen at a normal or oblique angle. The amount ofscattered light detected by the system generally depends on the opticalcharacteristics of the spot under inspection (e.g., the refractive indexof the spot), as well as any spatial variations within the spot (e.g.,uneven surface topologies). In the case of dark-field inspection, smoothsurfaces lead to almost no collection signal, while surfaces withprotruding features (such as patterned features or defects) tend toscatter much more strongly (sometimes up to six orders of magnitude ormore). Bright-field inspection systems direct light to a specimen at aparticular angle and measure the amount of light reflected from thesurface of the specimen at a similar angle. In contrast to dark-fieldsystems, the variations in the reflected signal collected by abright-field system are generally no more than about two orders ofmagnitude.

In addition, most inspection tools are designed to inspect eitherunpatterned or patterned semiconductor wafers, but not both. Since thetools are optimized for inspecting a particular type of wafer, they aregenerally not capable of inspecting different types of wafers for anumber of reasons. For example, many unpatterned wafer inspection toolsare configured such that all of the light collected by a lens (oranother collector) is directed to a single detector that generates asingle output signal representative of all of the light collected by thelens. Therefore, light scattered from patterns or features on apatterned wafer will be combined with other scattered light (e.g., fromdefects). In some cases, the single detector may become saturated, andas a result, may not yield signals that can be analyzed for defectdetection. Even if the single detector does not become saturated, thelight scattered from patterns or other features on the wafer cannot beseparated from other scattered light thereby hindering, if notpreventing, defect detection based on the other scattered light.

Tools used for inspecting patterned wafers generally employ at least twodetectors for improved spatial resolution. However, the detectors usedin patterned wafer inspection tools may also become saturated,especially when imaging with a dark-field system. As noted above,dark-field scattering signals obtained from a patterned wafer may varyby six orders of magnitude (or more) due to the variation in surfacetopology from smooth surface regions (which appear dark) to highlytextured regions (which appear bright). It is often difficult,especially with detection systems operating at high data rates, tocollect meaningful signals from both the very dark and the very brightareas of the substrate being inspected without “on-the-fly” adjustment.

Optical inspection tools may be limited in either detection range,detection sensitivity, or both. For example, inspection tools employinghigh-gain detectors to obtain higher detection range may be incapable ofdetecting smaller (i.e., low light) signals. On the other hand,inspection tools with lower-gain detectors may achieve greatersensitivity at the cost of reduced detection range. In other words,although lower gain detectors may be capable of detecting smallersignals, they may become saturated when larger signals are received.Other factors tend to limit the detection range, in addition to detectorgain. For example, further limitations may be imposed by theamplification circuitry or the fast analog-to-digital converters used toconvert the scattered output signals into a format suitable for signalprocessing.

Therefore, a need remains for improved circuits and methods forextending the detection range of a wafer inspection system. In addition,an improved inspection system would extend the detection range withoutsacrificing throughput or sensitivity. In some cases, the improvedinspection system may be used for inspecting both patterned andunpatterned wafers.

SUMMARY OF THE INVENTION

An inspection system may include, but is not limited to: an illuminationsubsystem for directing light to an inspection specimen comprising: apower attenuator subsystem configured for altering the power level of alight beam emitted by the illumination subsystem; and a powerattenuation control subsystem configured to provide control signals tothe power attenuator subsystem according to a detected level of lightscattering by the inspection specimen upon illumination by theillumination subsystem. A method for scatterometry inspection mayinclude, but is not limited to: directing light having a power level toan inspection specimen from a light source; detecting light scatteredfrom the specimen; and modifying a power level of one or moreintermediate light beams within the light source according to a level oflight scattering by the specimen upon illumination by the light source.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 is a block diagram of an exemplary inspection system including anillumination subsystem for directing light towards a specimen, adetection subsystem for detecting light scattered from the specimen, anda processor for detecting features, defects or light scatteringproperties of the specimen using the detected light;

FIG. 2A is a block diagram of an exemplary circuit included within thedetection subsystem of FIG. 1 for detecting the light scattered from thespecimen, according to a first embodiment of the invention;

FIG. 2B is a block diagram of an exemplary circuit included within thedetection subsystem of FIG. 1 for detecting the light scattered from thespecimen, according to a second embodiment of the invention;

FIG. 2C is a block diagram of an exemplary circuit included within thedetection subsystem of FIG. 1 for detecting the light scattered from thespecimen, according to a third embodiment of the invention;

FIG. 3 is a flow chart diagram of an exemplary method for inspecting aspecimen, in accordance with the first, second and third embodiments ofthe invention;

FIG. 4A is a block diagram of an exemplary circuit included within thedetection subsystem of FIG. 1 for detecting the light scattered from thespecimen, according to a fourth embodiment of the invention;

FIG. 4B is a block diagram of an exemplary circuit included within thedetection subsystem of FIG. 1 for detecting the light scattered from thespecimen, according to a fifth embodiment of the invention;

FIG. 5 is a block diagram of exemplary electronics that may be used forfiltering, digitizing and switching between the signals generated by thecircuits of FIGS. 4A and 4B;

FIG. 6 is a flow chart diagram of an exemplary method for inspecting aspecimen, in accordance with the fourth and fifth embodiments of theinvention;

FIG. 7 is a block diagram of another exemplary inspection systemincluding means for controlling the amount of incident laser powersupplied to a specimen under observation;

FIG. 8 is a block diagram of an exemplary embodiment of a laser powerattenuator, which may be included within the inspection system of FIG.7;

FIG. 9 is a block diagram of an exemplary embodiment of a laser powercontroller, which may be included within the inspection system of FIG.7;

FIG. 10 is a block diagram of another exemplary embodiment of a laserpower controller, which may be included within the inspection system ofFIG. 7;

FIG. 11 is a block diagram of yet another exemplary embodiment of alaser power controller, which may be included within the inspectionsystem of FIG. 7;

FIG. 12 is a flow chart diagram of a method for dynamically altering theamount of incident laser power supplied to a specimen under observation,so as to reduce thermal damage of large particles and extend themeasurement detection range of the inspection system of FIG. 7;

FIG. 13 is a block diagram of another exemplary inspection systemincluding means for controlling the amount of incident laser powersupplied to a specimen under observation;

FIG. 14 is a block diagram of an exemplary embodiment of a laser powerattenuator, which may be included within the inspection system of FIG.13;

FIG. 15 is a block diagram of an exemplary embodiment of a laser powerattenuator, which may be included within the inspection system of FIG.13; and

FIG. 16 is a chart detailing properties of various Pockel's Cellmaterials.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention generally relates to circuits, systems and methodsfor inspecting a specimen. In particular, the present invention relatesto circuits, systems and methods for reducing thermal damage to largeparticles by dynamically altering an incident beam power level appliedto the specimen during a surface inspection scan. In addition, thesystems and methods described herein may be used to extend themeasurement detection range of an inspection system by providing avariable-power inspection system.

The methods and systems described herein enhance defect detection byaddressing various limiting factors of measurement detection rangeincluding, but not limited to, detector saturation, amplifier saturationand the fixed bit range of analog-to-digital converters (ADC). Unlikesome currently used inspection methods, the inspection system describedherein is able to extend the measurement detection range whilemaintaining signal linearity and stability, and without employingadditional detectors, optics and electronic components, all of whichundesirably increase space consumption, complexity and cost of theinspection system.

Various embodiments are described herein for an optical inspectionsystem or tool that may be used for inspecting a specimen. The term“specimen” is used herein to refer to a wafer, a reticle, or any othersample that may be inspected for defects, features, or other information(e.g., an amount of haze or film properties) known in the art.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide and indium phosphide. Suchsubstrates may be commonly found and/or processed in semiconductorfabrication facilities.

In some cases, a wafer may include only the substrate, such as a virginwafer. Alternatively, a wafer may include one or more layers that may beformed upon a substrate. Examples of such layers may include, but arenot limited to, a resist, a dielectric material and a conductivematerial. A resist may include a resist that may be patterned by anoptical lithography technique, an e-beam lithography technique, or anX-ray lithography technique. Examples of a dielectric material mayinclude, but are not limited to, silicon dioxide, silicon nitride,silicon oxynitride, and titanium nitride. Additional examples of adielectric material include “low-k” dielectric materials such as BlackDiamond which is commercially available from Applied Materials, Inc.,Santa Clara, Calif., and CORAL commercially available from NovellusSystems, Inc., San Jose, Calif., “ultra-low k” dielectric materials,such as “xerogels,” and “high-k” dielectric materials, such as tantalumpentoxide. In addition, examples of conductive materials may include,but are not limited to, aluminum, polysilicon and copper.

One or more layers formed on a wafer may be “patterned” or“unpatterned.” For example, a wafer may include a plurality of dieshaving repeatable pattern features. Formation and processing of suchlayers of material may ultimately result in completed semiconductordevices. As such, a wafer may include a substrate on which not alllayers of a complete semiconductor device have been formed, or asubstrate on which all layers of a complete semiconductor device havebeen formed. The term “semiconductor device” may be used interchangeablyherein with the term “integrated circuit.” In addition, other devicessuch as microelectromechanical (MEMS) devices and the like may also beformed on a wafer.

A “reticle” may be a reticle at any stage of a reticle fabricationprocess, or a completed reticle that may or may not be released for usein a semiconductor fabrication facility. A reticle, or a “mask,” isgenerally defined as a substantially transparent substrate havingsubstantially opaque regions formed thereon and configured in a pattern.The substrate may include, for example, a glass material such as quartz.A reticle may be disposed above a resist-covered wafer during anexposure step of a lithography process such that the pattern on thereticle may be transferred to the resist. For example, substantiallyopaque regions of the reticle may protect underlying regions of theresist from exposure to an energy source.

Turning now to the drawings, it is noted that various Figures may not bedrawn to scale. In particular, the scales of some of the elements of thefigures are greatly exaggerated to emphasize characteristics of theelements. Similar elements shown in more than one figure have beenindicated using the same reference numerals.

FIG. 1 illustrates a system that may be used to perform the inspectionmethods described herein. The system shown in FIG. 1 illustrates ageneral optical configuration that can be used to inspect a specimenaccording to the methods described herein. The inspection systemincludes a dark-field optical subsystem. It will be obvious to one ofordinary skill in the art that the illustrated system may be altered inmany ways while still providing the capability to perform the methodsdescribed herein. In addition, it will be obvious to one of ordinaryskill in the art that the illustrated system may include variousadditional components that are not shown in FIG. 1 such as a stage, aspecimen handler, folding mirrors, polarizers, additional light sources,additional collectors, etc. All such variations are within the scope ofthe invention described herein.

The system illustrated in FIG. 1 includes an illumination subsystem. Theillumination subsystem is configured to direct light to a specimen. Forexample, the illumination subsystem includes light source 29. Lightsource 29 may include, for example, a laser, a diode laser, a heliumneon laser, an argon laser, a solid-state laser, a diode pumped solidstate (DPSS) laser, a xenon arc lamp, a gas discharging lamp, or anincandescent lamp. The light source may be configured to emit nearmonochromatic light or broadband light. In general, the illuminationsubsystem is configured to direct light having a relatively narrowwavelength band to the specimen (e.g., nearly monochromatic light orlight having a wavelength range of less than about 20 mm, less thanabout 10 nm, less than about 5 nm, or even less than about 2 nm).Therefore, if the light source is a broadband light source, theillumination subsystem may also include one or more spectral filtersthat may limit the wavelength of the light directed to the specimen. Theone or more spectral filters may be bandpass filters and/or edge filtersand/or notch filters.

The illumination subsystem also includes various beam forming andpolarization control optics 12. For example, the illumination subsystemmay include various optics for directing and supplying an incident beamto specimen 14 with, e.g., a particular spot size. If the light sourceis configured to emit light of various polarizations, the illuminationsubsystem may also include one or more polarizing components that mayalter the polarization characteristics of the light emitted by the lightsource. In some cases, the light directed to specimen 14 may be coherentor incoherent. The beam forming and polarization control optics 12 mayinclude a number of components, which are not shown in FIG. 1, such as abeam expander, folding mirrors, focusing lenses, cylindrical lenses,beam splitters, etc.

In some cases, the illumination subsystem may include a deflector (notshown). In one embodiment, the deflector may be an acousto-opticaldeflector (AOD). In other embodiments, the deflector may include amechanical scanning assembly, an electronic scanner, a rotating mirror,a polygon based scanner, a resonant scanner, a piezoelectric scanner, agalvo mirror, or a galvanometer. The deflector scans the light beam overthe specimen. In some embodiments, the deflector may scan the light beamover the specimen at an approximately constant scanning speed.

As shown in FIG. 1, the illumination subsystem may be configured todirect the beam of light to the specimen at a normal angle of incidence.In this embodiment, the illumination subsystem may not include adeflector since the normal incidence beam of light may be scanned overthe specimen by relative motion of the optics with respect to thespecimen and/or by relative motion of the specimen with respect to theoptics. Alternatively, the illumination subsystem may be configured todirect the beam of light to the specimen at an oblique angle ofincidence. The system may also be configured to direct multiple beams oflight to the specimen such as an oblique incidence beam of light and anormal incidence beam of light. The multiple beams of light may bedirected to the specimen substantially simultaneously or sequentially.

The inspection system of FIG. 1 includes a single collection channel.For example, light scattered from the specimen may be collected bycollector 16, which may be a lens, a compound lens, or any appropriatelens known in the art. Alternatively, collector 16 may be a reflectiveor partially reflective optical component, such as a mirror. Inaddition, although one particular collection angle is illustrated inFIG. 1, it is to be understood that the collection channel may bearranged at any appropriate collection angle. The collection angle mayvary depending upon, for example, the angle of incidence and/ortopographical characteristics of the specimen.

The inspection system also includes a detector 18 for detecting thelight scattered from the specimen and collected by collector 16.Detector 18 generally functions to convert the scattered light into anelectrical signal, and therefore, may include substantially anyphotodetector known in the art. However, a particular detector may beselected for use within one or more embodiments of the invention basedon desired performance characteristics of the detector, the type ofspecimen to be inspected and/or the configuration of the illuminationsubsystem. For example, if the amount of light available for inspectionis relatively low, an efficiency enhancing detector such as a time delayintegration (TDI) camera may increase the signal-to-noise ratio andthroughput of the system. However, other detectors such ascharge-coupled device (CCD) cameras, photodiodes, phototubes andphotomultiplier tubes (PMTs) may be used, depending on the amount oflight available for inspection and the type of inspection beingperformed. In at least one embodiment of the invention, aphotomultiplier tube is used for detecting light scattered from aspecimen.

The inspection system also includes various electronic components neededfor processing the scattered signals detected by detector 18. Forexample, the system shown in FIG. 1 includes amplifier circuitry 20,analog-to-digital converter (ADC) 22 and processor 24. Amplifier 20 isgenerally configured to receive output signals from detector 18 and toamplify those output signals by a predetermined amount. ADC 22 convertsthe amplified signals into a digital format suitable for use withinprocessor 24. In one embodiment, the processor may be coupled directlyto ADC 22 by a transmission medium, as shown in FIG. 1. Alternatively,the processor may receive signals from other electronic componentscoupled to ADC 22. In this manner, the processor may be indirectlycoupled to ADC 22 by a transmission medium and any interveningelectronic components.

In general, processor 24 is configured for detecting features, defects,or light scattering properties of the specimen using electrical signalsobtained from the single collection channel. The signals produced by thesingle collection channel are representative of the light detected by asingle detector (detector 18). The term “single detector” may be usedherein to describe a detector having only one sensing area, or possiblyseveral sensing areas (such as found, e.g., in a detector array ormulti-anode PMT). Regardless of number, the sensing areas of a singledetector are embodied within a single enclosure. In some cases, theinspection system described herein may be used for inspecting patterned,as well as unpatterned specimens. The processor may include anyappropriate processor known in the art. In addition, the processor maybe configured to use any appropriate defect detection algorithm ormethod known in the art. For example, the processor may use adie-to-database comparison or a thresholding algorithm to detect defectson the specimen.

The inspection system described herein provides more features, defects,or light scattering property information about specimens than otherinspection systems, which trade-off detection range for sensitivity (orvice versa). In other words, the inspection system described hereinprovides extended detection range (e.g., about 0 to about 3 orders ofmagnitude, or more) without sacrificing sensitivity. The improvedinspection system also maintains excellent signal linearity andstability, and does not require complex calibrations or additionaldetectors and optics to extend the detection range. The improvedinspection system achieves all this by addressing several factors, whichtend to limit the detection range of an inspection system. These factorsinclude, but are not limited to, detector saturation, amplifiersaturation and the fixed bit range of analog-to-digital converters. Thelimitations set by detector saturation will now be described in thecontext of photomultiplier tubes. It is recognized, however, that thegeneral concepts outlined below may be applicable to other types ofdetectors.

Photomultiplier tubes (PMTs) are often used as detectors when opticalsignals are dim (i.e., in low-intensity applications, such asfluorescence spectroscopy). A typical photomultiplier tube consists of aphotoemissive cathode (photocathode) followed by focusing electrodes, aplurality of dynodes (forming an electron multiplier) and an anode(forming an electron collector) in a vacuum tube. When light enters thePMT, the photocathode emits photoelectrons into the vacuum. The focusingelectrodes direct the photoelectrons towards the electron multiplierwhere electrons are multiplied by the process of secondary emission. Forexample, the photoelectrons are accelerated from the photocathode to thefirst dynode by an electric field. When they strike the dynode, theydislodge additional electrons to amplify the photoelectric signal. Thesesecondary electrons cascade towards the next dynode where they are againamplified. At the end of the dynode chain, the electrons are collectedby the anode to generate an electrical output signal in proportion tothe amount of light entering the PMT. The output signal produced at theanode is generally large enough to be measured using conventionalelectronics, such as a trans-impedance amplifier followed by ananalog-to-digital converter.

The process of secondary emission enables the photomultiplier tube toachieve high current amplification. In other words, a very smallphotoelectric current from the photocathode can be observed as a largeoutput current from the anode of the photomultiplier tube. Currentamplification (otherwise referred to as gain) is simply the ratio of theanode output current to the photoelectric current from the photocathode.The gain at each dynode is a function of the energy of the incomingelectrons, which is proportional to the electric potential between thatdynode and the previous stage. The total gain of the PMT is the productof the gains from all of the dynode stages. When a voltage (V) isapplied between the cathode and the anode of a photomultiplier tubehaving (n) dynode stages, the total gain becomes:G(V)αV ^(αn)  EQ. 1where, α is a coefficient determined by the dynode material andgeometric structure (typically in the range of 0.6 to 0.8).

In most cases, a photomultiplier tube will be operated at a singlepredetermined gain. For example, bias voltages may be generated for eachof the dynodes by connecting a string of voltage-divider resistorsbetween the cathode, all of the dynodes, the anode and ground. Theresistance, R, is used as a scaling constant and is typically the samefor all stages of the photomultiplier tube. A large negative voltage(typically −500 V to −1500 V) is then applied to the cathode, and thepotential is divided up evenly across the dynodes by the voltage-dividerresistor chain. Doing so enables each of the dynodes to be maintained atsuccessively less negative potentials, the difference between whichestablishes the intermediate dynode gain. Though the total gain of thephotomultiplier tube may be altered by changing the voltage applied tothe cathode, it is generally not desirable to do so. For example, thelarge voltages involved make it difficult to change the gain quickly,due to parasitic capacitances and the large resistor values needed tolimit power dissipation in the bias string. Therefore, most users decideon a tube gain in advance, set the appropriate cathode voltage and thenoperate the tube at that voltage throughout the measurement operations.

In this configuration, the detection range of the photomultiplier tubeis limited on the low end by the noise and gain characteristics of thetransimpedance amplifier and, on the high end, by the ability of thephotomultiplier tube to deliver anode current. In low-intensityapplications, the anode current is limited by space charge effectswithin the tube, bias string power consumption, and the consumablenature of the material coating the dynodes. In high-intensityapplications, the photomultiplier tube is limited by saturation of theanode, and sometimes, the cathode. For example, the photomultiplier tubemay provide inaccurate results when relatively large amounts of lightcause the anode (or cathode) to become saturated. In the followingembodiments, the present invention addresses anode saturation as alimiting factor to the detection range of an inspection system. Asdescribed in more detail below, the present invention avoids measurementinaccuracies and extends the detection range of a PMT detector byproviding circuits and methods designed, in one aspect, for avoidinganode saturation.

FIG. 2A illustrates one embodiment of a circuit 30 that may be used fordetecting light scattered from a specimen. As such, circuit 30 may beincorporated within the inspection system of FIG. 1 as detector 18. Inthe embodiment of FIG. 2A, circuit 30 includes a photomultiplier tube(PMT) 32 having a cathode 34, a plurality of dynodes 36 and an anode 38.Though shown having only 8 dynodes, PMT 32 may include substantially anyappropriate number of dynodes, with typical numbers ranging betweenabout 8 and about 20. PMT 32 is also illustrated in FIG. 2A as a head-onphotomultiplier tube, and in particular, a linear-focused head-on PMToperated in a transmission mode. However, one of ordinary skill in theart would recognize how the inventive aspects described herein may beapplied to other modes and/or types of PMTs. For example, PMT 32 may bealternatively formed in a side-on configuration and/or operated inreflection mode. In addition to the linear-focused PMT shown in FIG. 2A,the inventive aspects could be applied to other types of PMTs including,but not limited to, circular-cage type, box-and-grid type, Venetianblind type, and mesh type.

Like conventional PMT circuits, circuit 30 includes a voltage-dividerchain 40 with impedance elements (e.g., Z₁-Z₉) coupled across thecathode, most of the dynodes and the anode. The impedance elements mayinclude resistors of equal resistance, a tapered resistance or even acombination of resistors and capacitors, as is known in the art.Therefore, when a high negative voltage (V_(S1)) is applied to thecathode, the potential may be divided up evenly across all of thedynodes, or somewhat unevenly so that dynodes in the middle of the chainexperience less gain. When light (hv) enters the PMT, cathode 34 emitsphotoelectrons which cascade through dynode chain 36 to produce anamplified photoelectric current (I_(A)) at anode 38. The current outputfrom anode 38 is converted into a voltage (V_(A)) by current-voltageconverter 42. In most cases, converter 42 may be an operationalamplifier with feedback (Z_(F)) and load (Z_(L)) impedances, such thatthe voltage (V_(A)) generated at the output of the PMT is related to theanode current by:V _(A) =I _(A)*(Z _(F) /Z _(L))  EQ. 2

Unlike conventional PMT circuits, circuit 30 includes additional circuitelements for measuring an intermediate dynode voltage (V_(D)), inaddition to the anode voltage (V_(A)). In one example, the intermediatedynode voltage may be obtained by supplying the photoelectric current(I_(D)) generated at one of the dynode stages 36 to an additionalimpedance element (Z_(D)), such that:V _(D) =I _(D)(Z _(D))  EQ. 3

In most cases, the intermediate dynode voltage will be less than theanode voltage, since the dynode voltage represents the photoelectriccurrent before the current is fully amplified at the anode. As describedin more detail below, the intermediate dynode voltage may be used inplace of the anode voltage for detecting defects on a specimen. Forexample, the intermediate dynode voltage may be used for detectingdefects once the photoelectric current at the anode causes the anode tosaturate. Switching logic 44 is included within circuit 30 formonitoring the anode voltage and switching to the intermediate dynodevoltage once the anode current reaches saturation.

The level at which anode 38 becomes saturated may depend on severalfactors including, but not limited to, the material composition andgeometrical structure of the anode, the high voltage between the lastdynode and the anode, and the configuration of the voltage-dividerchain. As known in the art, an element may become “saturated” when anyfurther change in input no longer results in an appreciable change inthe output signal generated by that element. In some cases, anode 38 mayprovide a highly linear anode current up to about 10 mA. The anode maythen become saturated if the amount of light entering the PMT causes theanode current to rise above approximately 10 mA. It is noted, however,that smaller or larger saturation levels may be appropriate in otherembodiments of the invention. In some cases, for example, largersaturation levels may be appropriate when the anode current remainssubstantially linear up to about 100 mA.

To avoid anode saturation, switching logic 44 may switch to theintermediate dynode voltage (V_(D)) once the anode current (I_(A))approaches, reaches or surpasses the saturation level. For example,switching logic 44 may use the anode saturation level, or a valueslightly above or slightly below such level, as a predeterminedthreshold level. During circuit operation, switching logic 44 monitorsthe anode current (I_(A)) while supplying the anode voltage (V_(A)) todownstream processing components (e.g., ADC 22 and processor 24) as anoutput signal (V_(OUT)) of the detector. When the anode current reachesthe predetermined threshold level, switching logic 44 uses theintermediate dynode voltage (V_(D)) as the detector output signal(V_(OUT)). Regardless of whether the anode or dynode current is used,the detector output signal forwarded by switching logic 44 may beamplified by ADC 22 by a substantially consistent amount.

Switching logic 44 may be implemented as hardware, software, or acombination of both (i.e., firmware). As such, switching logic 44 may belocated within or adjacent to circuit 30 as a combination of logicelements, which perform the functionality embodied within switchinglogic 44. On the other hand, the functionality may be implemented withprogram instructions, which may be stored and executed “on-chip” or“off-chip.” For example, switching logic 44 may be implemented as adigital signal processing (DSP) chip coupled to circuit 30, or asprogram instructions executed by processor 24. Otherconfigurations/implementations of switching logic 44 may be possible andwithin the scope of the invention. For example, switching logic 44 maypreferably monitor the anode voltage (V_(A)) instead of the anodecurrent (I_(A)), and thus, may not be coupled for receiving the anodecurrent, as shown in FIG. 2A.

As noted above, the intermediate dynode voltage is obtained from one ofthe plurality of dynodes 36. The particular dynode selected maygenerally depend on a desired gain differential between the anode anddynode voltages. For example, each of the dynodes is supplied with aparticular bias voltage generated by voltage-divider chain 40 and thenegative high-voltage power supply (V_(S1)) coupled thereto. Thehigh-voltage power supply may generally be chosen to provide aparticular amount of overall (or “tube”) gain. In most cases, each ofthe dynodes 36 may be supplied with a successively less negativepotential, due to the increasing series resistance encountered along thevoltage-divider chain. When supplied with a negative power supply ofabout −1000V, for example, the impedance elements (e.g., Z₁ to Z₉)within voltage divider chain 40 may be configured to supply successivelyless negative potentials to the dynode chain, such that −900V issupplied to the first dynode, −800V is supplied to the second dynode,and so forth. The potential difference between voltages supplied to aselected dynode and the anode determine the gain differentialtherebetween.

A desired gain differential, and thus, a desired dynode may be selectedbased on variations in the light intensity entering the PMT. Forhigh-intensity applications, a dynode near the beginning of the chainmay be selected to produce a larger gain differential between the dynodeand anode voltages. Such a large gain differential may allowsubstantially larger amounts of light to be detected after the anodebecomes saturated. On the other hand, dynodes further down the chain maybe selected to produce smaller gain differentials, which may suffice formeasuring a smaller range of light intensities. In some embodiments, adynode near the middle of the dynode chain may be selected for producinga medium gain signal. As such, the circuits and methods described hereinmay provide the user with a range of possible gain differentials.

This range may be approximately equal to about G^(((n-m)/n)), where G isthe total PMT Gain (cathode to anode), m is the position of the selectedintermediate dynode (counting from the cathode), and n is the totalnumber of dynodes.

In the embodiment of FIG. 2A, the selected dynode is disconnected fromthe voltage-divider chain, and the intermediate dynode voltage ismeasured across the additional impedance element (Z_(D)). The additionalimpedance element may be implemented with substantially any passive oractive element, although a simple resistance may be preferred. Ingeneral, the size of the additional impedance element should be chosenso that the signal measured there across is substantially equal to thesignal level at the anode. As shown in FIG. 2A, an additional powersupply (V_(S2)) may also be needed for measuring the intermediate dynodevoltage. In some cases, the additional power supply level may be set toprovide the potential, which would have been supplied to the selecteddynode if the dynode had not been disconnected from the voltage-dividerchain.

FIG. 2B illustrates an alternative embodiment of circuit 30. Similarreference numerals are used to denote similar features between theembodiments shown in FIGS. 2A and 2B; therefore, detailed description ofsuch features will not be repeated for purposes of brevity. Similar toFIG. 2A, the embodiment shown in FIG. 2B includes a PMT detector 32having a cathode 34, a plurality of dynodes 36 and an anode 38. In someembodiments, the photoelectric current (I_(A)) generated at the anodemay be supplied to current-voltage converter 42 and switching logic 44.Once the anode current (I_(A)) reaches a predetermined threshold level(associated with anode saturation), the switching logic may switch tosupplying the intermediate dynode voltage (V_(D)) to downstreamprocessing components. In other embodiments, the anode voltage (V_(A))may be monitored instead of the anode current (I_(A)). In suchembodiments, switching logic 44 may not be coupled for receiving theanode current, as shown in FIG. 2B.

The embodiments shown in FIGS. 2A and 2B differ in the manner in whichthe intermediate dynode voltage is produced. Instead of a singlevoltage-divider chain 40, the embodiment of FIG. 2B divides the chaininto a first portion 40 and a second portion 40 b. The first portion 40a is supplied with a first power supply voltage (V_(S1)), while thesecond portion 40 b is supplied with a second power supply voltage(V_(S2)). In this configuration, the intermediate dynode voltage can bemeasured across the last impedance element (Z₅) in the first portion 40a. The advantage of this configuration is that saturation of the anodeand the portion of the voltage-divider chain (e.g., portion 40 b)supplying current to the higher dynodes will not affect the portion ofthe chain (e.g., portion 40 a) supplying the lower dynodes.

FIG. 2C illustrates yet another alternative embodiment of circuit 30.Like before, similar reference numerals are used to denote similarfeatures between the embodiments shown in FIGS. 2A-2C; therefore,detailed description of such features will not be repeated for purposesof brevity. Similar to FIGS. 2A and 2B, the embodiment shown in FIG. 2Cincludes a PMT detector 32 having a cathode 34, a plurality of dynodes36 and an anode 38. In some embodiments, the photoelectric current(I_(A)) generated at the anode may be supplied to current-voltageconverter 42 and switching logic 44. Once the anode current (I_(A))reaches a predetermined threshold level (associated with anodesaturation), the switching logic may switch to supplying theintermediate dynode voltage (V_(D)) to downstream processing components.In other embodiments, the anode voltage (V_(A)) may be monitored insteadof the anode current (I_(A)). In such embodiments, switching logic 44may not be coupled for receiving the anode current, as shown in FIG. 2C.

FIG. 2C illustrates yet another manner in which the intermediate dynodevoltage (V_(D)) may be produced. For example, circuit 30 may includeoperational amplifier circuit 46 for modifying the gain of theintermediate dynode voltage (V_(D)). The operational amplifier may havea pair of inputs, each coupled to a different terminal of the additionalimpedance element (Z_(D)). In some cases, load and feedback impedancesmay be coupled to at least one of the operational amplifier inputs. Inthis configuration, the intermediate dynode voltage measured across theadditional impedance element (Z_(D)) can be modified to produce a signalwith somewhat higher or lower gain. For example, the configuration shownin FIG. 2C could be used to bring the intermediate dynode voltage from ahigh negative potential to a near ground level, which is the same levelas the anode output.

FIG. 3 is a flow chart diagram illustrating an exemplary method forinspecting a specimen using the inspection system of FIG. 1 and thecircuits shown in FIGS. 2A and 2B. Various method steps set forth inFIG. 3 may be performed by components included within the inspectionsystem, although certain steps may be performed by a user of theinspection system.

In some embodiments, the method may begin by selecting a dynode (in step50) to be used for measuring an intermediate dynode voltage. In somecases, a particular dynode may be selected by a user of the inspectionsystem to provide a desired gain differential between the selecteddynode and the anode of the PMT. The desired gain differential may be anassumption, an educated guess or a predetermined amount based onexpected or previous levels of scattered light intensity. In othercases, the particular dynode may be automatically selected by aninspection system component (e.g., processor 24) based on the expectedor previous levels of scattered light intensity.

In step 52, light is directed to a specimen under observation. As notedabove, the term “specimen” may refer to a wafer, a reticle or any othersample that may be inspected for defects, features, or other information(e.g., an amount of haze or film properties) known in the art. In theembodiments described herein, the system used for inspecting thespecimen is a dark-field optical inspection system, which measuresscattered rather than reflected light. Therefore, the method maycontinue by detecting light scattered from the specimen (in step 54)using the anode current from a photomultiplier tube (PMT). The PMT maybe configured as shown in FIG. 2A or 2B. As shown in FIG. 3, the anodecurrent may be used for detecting the scattered light, as long as theanode current remains below a predetermined threshold level. Thisthreshold level may be set manually by a user of the system, orautomatically by a system component, based the type/configuration of thePMT. In most cases, the predetermined threshold level may be close orequal to a saturation level associated with the anode.

If the anode current remains below the predetermined threshold (in step56), features, defects and/or light scattering properties of thespecimen may be detected using the anode current (in step 58).Otherwise, the current from the selected dynode may be used for suchdetecting (in step 60). Using the dynode current may allow significantlylarger amounts of scattered light to be measured at the dynode than canbe measured at the anode. This is due to there being less amplificationat the dynode, which may enable approximately 1 to 1000 times largersignals to be measured at the dynode (using, e.g. a middle dynode)without dynode saturation. To maintain accuracy, however, the dynodecurrent must be multiplied by a calibration ratio approximately equal tothe gain ratio of the anode and intermediate dynode. In reference toFIG. 1, the calibration ratio may be applied to the dynode current bysystem processor 24 or another system component (e.g., an analog or dataprocessing board). Regardless of how the calibration ratio is applied,use of the calibration ratio enables the dynode current to be used fordetecting defects, as if it had been generated at the anode.

In some cases, the method may end at step 60. In other cases, however,the method may include additional steps (e.g., steps 62, 64 and 66) ifthe dynode originally selected (in step 50) is for some reasoninsufficient for current or subsequent measurement purposes. Forexample, a relatively low gain dynode may be insufficient for detectingextremely low-light signals. On the other hand, a relatively high gaindynode may saturate when extremely high intensity light is supplied tothe PMT. If the originally selected dynode is determined to beinsufficient (in opt. step 62), an alternative dynode may be selected(in opt. step 64) and used for detecting the features, defects and otherlight scattering properties of the specimen (in opt. step 66). Thealternative dynode may be selected by a user or component of theinspection system during the measurement operation currently underway,or in preparation for a next measurement operation to be performed.

FIGS. 1-3 illustrate exemplary inspection systems, circuits and methodsfor improving defect detection by increasing the measurement detectionrange of a PMT detector, while maintaining detection signal linearityand stability. For example, the anode current may be used for detectingscattered light on the low-intensity side, whereas the intermediatedynode current may be used for detecting on the high-intensity side.Unlike conventional PMT detectors, which have limited detection rangedue to the inevitable anode saturation, the PMT detector describedherein extends the measurement detection range (on-the-high-intensityside) by switching to lower gain dynode currents once the anode becomessaturated. In this manner, the PMT detector described herein may be usedto detect substantially more features, defects or light scatteringproperties of the specimen by dynamically switching between a highersensitivity, lower saturation detection signal (obtained from the anodecurrent) and a lower sensitivity, higher saturation detection signal(obtained from the dynode current). The anode current may be used fordetecting features, such as small defects with low scattering intensity,whereas large, highly scattering defects may be detectable with thedynode current.

In addition to extended detection range, the PMT detector describedherein maintains detection signal linearity and stability by avoidinganode saturation with a simple, yet highly effective solution. Inaddition, the PMT detector of the present invention avoids the elaboratecalibrations and complex circuitry often required in prior art designsto compensate for non-linear and transient effects, which may beintroduced when attempting to change the PMT gain “on-the-fly.”Furthermore, the present invention provides these advantages while usinga single detector, and thus, avoids the additional optics and controlcircuitry required when using multiple detectors to extend themeasurement detection range. As such, the present invention may providesignificant savings in both space consumption and cost.

FIGS. 4-6 illustrate exemplary circuits, systems and methods forovercoming the detection range limitations typically set by amplifiersaturation and the fixed bit range of analog-to-digital converters(ADC). Though relatively few embodiments are shown, one skilled in theart will readily understand how the various concepts described hereincould be applied to produce alternative embodiments with similarfunctionality.

FIGS. 4A and 4B illustrate exemplary embodiments of detector andamplifier circuitry that may be used within an inspection system fordetecting light scattered from a specimen. The detector circuitry 70shown in FIGS. 4A and 4B includes only one photodetector for detectingthe light scattered from the specimen and for converting the light intoan electrical signal. As noted above, a “single detector” may have onlyone sensing area or several sensing areas embodied within a singleenclosure. Therefore, the detectors shown in FIGS. 4A and 4B may includea single detector with only one sensing area, or a single detector arraywith multiple sensing areas. In general, detector circuitry 70 mayinclude substantially any technology that is suitable for detectinglight scattered from a specimen. Exemplary detectors include, but arenot limited to, a photodiode, a phototube, a photomultiplier tube (PMT),a time delay integration (TDI) camera and a charge-coupled device (CCD)camera.

Amplifier circuitry 72 is generally configured for producing of a pairof output signals in response to the electrical signal generated bydetector circuitry 70. For this reason, amplifier circuitry 72 may bereferred to herein as a “dual-output amplifier.” In the embodimentsshown, amplifier circuitry 72 is configured for generating ahigh-resolution (high gain) output signal and a low-resolution (lowgain) output signal from the electrical signal. As described in moredetail below, the high-resolution signal can be used for detectingfeatures, defects and/or light scattering properties of a specimen whenthe scattered light falls within a low-intensity range. If thehigh-resolution signal becomes saturated, the low-resolution signal canbe used for detecting additional features or properties of the specimenthat tend to scatter more strongly (i.e., when the scattered light fallswithin a high-intensity range). Means are provided below (e.g., in FIGS.5-6) for dynamically switching between the high-resolution andlow-resolution signals during an inspection system scan of a specimen,thereby extending the detection range of the inspection system byovercoming the limitations typically set by conventional amplifier andADC circuitry.

A first embodiment of a dual-output amplifier 72 is shown in FIG. 4A asincluding a pair of operational amplifiers (74, 76) and a voltagedivider network implemented, e.g., with resistors R 1 and R 2.Operational amplifiers (or “op amps”) 74 and 76 are configured asvoltage followers with negative feedback. The positive terminals (+) ofoperational amplifiers 74 and 76 are coupled for receiving thephotodetector current (I_(s)) generated by detector 70 multiplied bysome resistive value. In the embodiment of FIG. 4A, the photodetectorcurrent is multiplied by the value of resistor R₂ for op amp 76, and thevalue of resistors R₁ and R₂ for op amp 74. Because of the voltagefollower configuration, the voltages present at the output terminals ofthe op amps is substantially equal to those supplied to their positiveterminals. In this configuration, dual-output amplifier 72 may generatea high-resolution output signal (V_(H)) and a low-resolution output(V_(I)) signal substantially equal to:V _(H) =V ₁ =I _(S)*(R ₁ +R ₂)  and EQ. 4V _(L) =V ₂ =I _(S) *R ₂  EQ. 5

The values for resistors R₁ and R₂ are selected to provide the outputsignals with substantially different gains. In some cases, for example,the value for resistor R₁ may be 15 times larger than that of resistorR₂ to generate a high-resolution output signal (V_(H)) with 16 timesmore gain than the low-resolution output signal (V_(I)). In oneembodiment, resistor R₁ may have a value of about 7.5 kΩ and resistor R₂may have a value of about 500Ω.

In the example provided above, the values of R₁ and R₂ are selected toincrease the gain differential (and thus, the detection range of thecircuit) by a factor of about 2 to about 16. Other values may beselected to produce the same amount of gain, or to provide more or lessgain, in other embodiments of the invention. For example, the resistorvalues may be selected so as to increase the gain differential (andthus, the detection range of the circuit) from about 2 to about 1024times the initial range provided by the photodetector alone. The amountby which the detection range can be extended generally depends on theresolution of the downstream analog-digital converter (e.g., ADC 22,FIG. 1) and the desired resolution at the switching point between thehigh-resolution and low-resolution signals. For example, the gaindifferential could be extended by a factor of about 1024 with a 14-bitconverter (16383 ADC max) and a desired overlap resolution of 16 ADC.

Another embodiment of a dual-output amplifier 72 is shown in FIG. 4B. Inthis embodiment, amplifier 72 includes at least three operationalamplifiers (78, 80, 82). The first operational amplifier (78) is coupledfor receiving the photodetector current (I_(S)) at a negative terminal(−) and a ground potential at a positive terminal (+) of the op amp.Resistor R₁ is placed in the negative feedback of op amp 78 forgenerating an output voltage substantially equal to:V _(N) =−I _(S) *R ₁  EQ. 6

The second and third operational amplifiers (80, 82) are coupled forgenerating a pair of output signals based on the nodal voltage (V_(N))provided by op amp 78. For example, resistors R₂ and R₃ may be includedwithin op amp 80 for generating a high-resolution output signal (V_(H))substantially equal to:V _(H) =−V _(N) [R ₃ /R ₂]  EQ. 7

Likewise, resistors R₄ and R₅ may be included within op amp 82 forgenerating a low-resolution output signal (V_(L)) substantially equalto:V _(L) =−V _(N) [R ₅ /R ₄]  EQ. 8

As in the above embodiment, the values for resistors R₁, R₂, R₃, R₄, andR₅ may be selected to provide the output signals with substantiallydifferent gains. In one embodiment, the values for resistors R₃ and R₅may be set equal to one another, such that EQ. 8 becomes:V _(L) =−V _(N) [R ₃ /R ₄]  EQ. 9

Now, the values for resistors R₂ and R₄ may be selected to provide adesired gain differential. In one embodiment, the value for resistor R₂may be 16 times less than that of resistor R₄ to generate ahigh-resolution output signal (V_(H)) with 16 times more gain than thelow-resolution output signal (V_(L)). For example, resistor R₂ may havea value of about 62.5Ω, resistors R₁ and R₄ (which may be equal in someembodiments) may have a value of about 1 kΩ, and resistors R₃ and R₅ mayhave a value of about 500Ω. Other values may be selected to produce thesame amount of gain, or to provide more or less gain, in otherembodiments of the invention. For example, the resistor values may beselected so as to increase the gain differential (and thus, thedetection range of the circuit) from about 2 to about 1024 times theinitial range provided by the photodetector alone. In some embodiments,one or more additional amplifier circuits may be coupled in series withop amps 80 and 82 to increase/decrease the gain differential of thehigh-resolution and low-resolution output signals.

The high-resolution and low-resolution output signals generated bydual-output amplifier 72 (as shown, e.g., in FIG. 4A or FIG. 4B) aresupplied to separate processing channels (84, 90) where they arefiltered and converted into a pair of digital signals. As shown in FIG.5, for example, the high-resolution signal (V_(H)) generated bydual-output amplifier 72 may be supplied to processing channel 84 whereit is filtered by analog filter 86 and digitized by analog-to-digitalconverter (ADC₁) 88. Likewise, the low-resolution signal (V_(L))generated by dual-output amplifier 72 may be supplied to processingchannel 90 where it is filtered by analog filter 92 and digitized byanalog-to-digital converter (ADC₂) 94. Analog filters 86 and 92 may bean anti-aliasing filter such as a Butterworth filter, a Bessel filter,and so on. Substantially any N-bit analog-to-digital converter (88, 94)may be used for converting the high-resolution and low-resolution analogsignals into corresponding digital signals. The gain differentialbetween the analog signals causes the high-resolution digital signal tobe somewhat larger in value than the low-resolution digital signal.

At least one of the digital signals may be supplied to a downstreaminspection system component (such as processor 24 of FIG. 1) for furtherprocessing. For example, and as shown in FIG. 5, multiplexor 96 may becoupled for selecting either the high-resolution digital signal fromprocessing channel 84, or the low-resolution digital signal fromprocessing channel 90. The selection is controlled by switching logic 98which, in most cases, may be coupled to processing channel 84 formonitoring the high-resolution digital signal.

As noted above, each of the ADCs may be capable of handling up toN-number of digital bits. In some cases, switching logic 98 may selectthe high-resolution signal for output as long as its digital valueremains below a predetermined threshold value associated with the N-bitADCs (e.g., 2⁸=256 for an 8-bit ADC). Once the high-resolution signalreaches the predetermined threshold value, switching logic 98 may selectthe low-resolution signal for output. In this configuration, theinspection system may detect features, defects or light scatteringproperties of the specimen using the larger (i.e., high resolution)digital signal for detecting “low-scattering” features (such as, e.g.,small particles or surface defects). Once the predetermined thresholdvalue is reached, the smaller (i.e., low resolution) digital signal maybe used for detecting features that tend to scatter more strongly (suchas, e.g., large particles or surface defects). A minimum resolution ismaintained at the “switch point” between the high-resolution andlow-resolution signals by selecting a “predetermined threshold value,”which is less than the max ADC output (2″). For example, a maximumthreshold value substantially less than or equal to 2^(N-1) may beselected to maintain sufficient resolution at the switch point.

As shown in FIG. 5, switching logic 98 may be implemented as acombination of logic and/or storage elements which, when combined,perform the functionality embodied within switching logic 98. In otherembodiments, the functionality may be implemented with programinstructions, which may be stored and executed “on-chip” or “off-chip.”For example, switching logic 98 may be implemented as a digital signalprocessing (DSP) chip coupled to processing channels 84 and 90, or asone or more program instructions executed by processor 24. In the lattercase, each of the digital signals may instead be supplied to processor24 and multiplexor 96 may be removed from the block diagram of FIG. 5.Signal selection may then take place via program execution withinprocessor 24. Other configurations/implementations of switching logic 98may be possible and within the scope of the invention.

FIG. 6 is a flow chart diagram illustrating an exemplary method forinspecting a specimen using the inspection system of FIG. 1 and thecircuits and systems shown in FIGS. 4 and 5. Various method steps setforth in FIG. 6 may be performed by components included within theinspection system, although certain steps may be performed by a user ofthe inspection system.

In some embodiments, the method may begin by directing light to aspecimen under observation (in step 100). As noted above, the term“specimen” may refer to a wafer, a reticle or any other sample that maybe inspected for defects, features, or other information (e.g., anamount of haze or film properties) known in the art. In the embodimentsdescribed herein, the system used for inspecting the specimen is adark-field optical inspection system, which measures scattered ratherthan reflected light. Therefore, the method may continue by detectinglight scattered from the specimen (in step 102). Unlike some prior artinspection systems, which attempt to extend the detection range by usingmultiple detectors, the systems and methods described herein use onlyone detector for such detecting. The single detector used herein mayinclude substantially any photodetector commonly used for detectingscattered light and for converting the light into an electrical signal.

In step 104, the electrical signal generated by the detector isconverted into a pair of disproportionately amplified signals. Forexample, a dual-output amplifier similar to those shown in FIGS. 4A-4Bmay be used for generating a first signal and a second signal inresponse to the electrical signal. In some cases, the first signal mayhave a higher resolution (higher gain) than the second signal fordetecting substantially lower levels of scattered light. On the otherhand, the second signal may have a lower resolution (lower gain) thanthe first signal for detecting substantially higher levels of scatteredlight. In this manner, the lower levels of light detected by the firstsignal may fall within a first detection range, whereas the higherlevels of light detected by the second signal fall within a seconddetection range. To maintain sufficient resolution at the “switchpoint,” the second detection range may at least partially overlap thefirst detection range. As noted above, this may be achieved by selectinga predetermined threshold value that is less than a maximum value (e.g.,2^(N)) associated with the N-bit analog-to-digital converters 88, 90. Insome cases, ADCs 88 and 90 may each have a different number of bits,such as N and M, where one is larger than the other. In such a case,sufficient resolution can still be maintained at the switch point byselecting threshold values, which are less than the maximum values(e.g., 2^(N) and 2^(M)) associated with N-bit ADC 88 and M-bit ADC 90.

In most cases, the first (high resolution) signal may be used fordetecting features, defects or light scattering properties of thespecimen until the first signal reaches the predetermined thresholdvalue (in step 106). However, once the first signal reaches thepredetermined threshold value (in step 108), the second (low-resolution)signal may be used for detecting the features, defects or lightscattering properties of the specimen (in step 110). For example, whilethe first signal is being used for detecting, switching logic 98 maycompare a digital value of the first signal to the predeterminedthreshold value. Switching logic 98 may subsequently switch to thesecond signal once the digital value of the first signal reaches and/orsurpasses the predetermined threshold value.

FIGS. 4-6 illustrate exemplary inspection systems, circuits and methodsfor improving defect detection by increasing the measurement detectionrange of the amplifier and analog-digital circuitry included within aninspection system. For example, a dual-output amplifier may be used forgenerating high-resolution and low-resolution output signals from theelectrical signal provided by a single photodetector. The systems,circuits and methods described herein avoid saturating the amplifier andanalog-digital circuitry by dynamically switching between thehigh-resolution and low-resolution output signals during an inspectionscan. This essentially increases the measurement detection range andenables more features, defects or light scattering properties of thespecimen to be detected by extending the range of defect sizes that canbe detected with the two output signals. In one example, thehigh-resolution signal may be used for detecting defects in the range ofabout 40 nm to about 155 nm, while the low-resolution signal may be usedfor detecting defects in the range of about 80 nm to about 500 nm. Otherembodiments may be capable of detecting somewhat smaller or largerdefects. Factors that determine the size of defects that can be detectedwith the present system and method include, but are not limited to,laser power, wavelength, surface topology, spot-size, polarization,collection angles, detector efficiency, and noise. Overlapping thedetection ranges ensures that there will be sufficient resolution at the“switch point” between the high and low-resolution signals.

The systems, circuits and methods described herein also avoid theadditional space consumption and costs associated with conventionalinspection systems, which attempt to extend measurement detection rangeby using expensive optics to split the scattered light amongst multipledetectors. By using only one photodetector, the present invention mayalso improve the sensitivity of the detected signal by avoiding thesignal-to-noise drop that would occur if the scattered light were splitamongst multiple detectors.

In addition to the embodiments shown in FIGS. 4-6, the conceptsdescribed herein could be extended to more than two amplifier outputs.For example, the amplifier designs shown in FIGS. 4A and 4B could bemodified to generate three or more output signals (e.g., a highresolution, medium resolution and low-resolution signal) from theelectrical signal produced by detector 70. If a PMT detector similar tothose shown in FIGS. 2A and 2B is used to implement detector 70, thesystem shown in FIG. 5 could also be modified to switch to anintermediate dynode voltage if, e.g., the low-resolution signal becomessaturated. Other configurations and/or implementations may be possibleand, thus, are considered within the scope of the invention.

FIG. 7 illustrates another example of an inspection system that can beused to perform a method for inspecting a specimen, as described herein.In particular, FIG. 7 illustrates one example of an inspection systemthat may be used to reduce and/or prevent thermal damage to a specimenduring a surface inspection scan. Thermal damage is commonly seen inprior art systems when the incident laser light directed to the specimenis absorbed and inadequately dissipated by a feature on the specimen(such as large particles or defects). In addition to preventing thermaldamage, the inspection system of FIG. 7 can be used to provide yetanother means for extending the measurement detection range of aninspection system.

FIGS. 7-15 illustrate exemplary circuits, systems and methods forreducing particle damage during surface inspection scans, which usehigh-power laser-based inspection systems, by dynamically reducing theincident laser power before scanning over large, highly scatteringparticles. Though relatively few embodiments are shown, one skilled inthe art will readily understand how the various concepts describedherein could be applied to produce alternative embodiments with similarfunctionality.

In high-power laser-based inspection systems, the power density of theincident laser beam typically ranges between about 1 kW/cm² to about1000 kW/cm². Unfortunately, particle damage often occurs during surfaceinspection scans with high power density laser beams, due to the rapidpower transfer from the laser beam to a particle (or a portion of aparticle) on the specimen. Particles not capable of dissipating largeamounts of power tend to warm up quickly, and often explode due toinsufficient power dissipation. For example, organic materials (such asphotoresist particles) tend to dissipate significantly less power thaninorganic materials (such as metallic particles), and therefore, tend toexperience more damage. Unfortunately, exploded particles lead todebris, which can spread a large area of contamination over thespecimen.

Methods that may be used to reduce thermal damage include methods forscanning the entire wafer at a reduced laser power (e.g., a factor of 10less power) or an increased spot size (e.g., 2.5× to 5.7× larger spotsize) than the power or spot size known to inflict damage. Anothermethod may exclude the wafer center from inspection by blocking aportion of the incident laser beam. This method reduces thermal damageby eliminating the high power level typically supplied to the centerregion of the wafer. However, these methods either reduce sensitivity byreducing the signal-to-noise ratio (i.e., when reducing the laser poweror increasing the spot size), or result in incomplete inspection of thewafer (i.e., when excluding the center). As such, the above-mentionedmethods often miss defects on the wafer, due to poor sensitivity oroutright exclusion.

On the contrary, the inventive concepts described herein are based onthe observation that larger particles (typically >5 μm) are more likelyto be damaged by the incident laser beam than smaller particles. Forexample, larger particles have more surface area, and as such, tend toabsorb significantly more power than smaller particles having lesssurface area. Larger particles also tend to scatter significantly morelight than smaller particles, due to larger surface area and/orincreased surface irregularities. For example, the amount of lightscattered from a particle of radius, R, is relatively proportional tothe particle radius raised to the sixth power (i.e., R 6).

The inventive concepts described herein exploit the highly scatteringproperties of large particles to reduce thermal damage during a surfaceinspection scan. As set forth in more detail below, thermal damage maybe avoided by detecting the presence of a large particle and reducingthe incident laser beam power before a main portion of the beam reachesthe large particle. In one embodiment, the power reduction may beprovided by a fast laser power attenuator, which can be engaged toreduce the incident laser power to “safe” levels when scanning overlarge particles. The laser power attenuator can be disengaged tomaintain (or increase) the incident laser power at “full” power whenscanning lower-scatter portions of the wafer.

Turning to the drawings, FIG. 7 illustrates an inspection system similarto the system shown in FIG. 1 and described above. Elements common toboth FIGS. 1 and 7 are indicated with similar reference numerals, thedescription of which will not be repeated herein. Unlike the inspectionsystem of FIG. 1, the inspection system shown in FIG. 7 may be specificto high-power optical inspection systems. As such, light source 29 mayinclude any number of light sources with output power densities rangingfrom about 1 kW/cm² to about 1000 kW/cm². Examples of potentialhigh-power, laser-based sources that may be used for light source 29include, but are not limited to, a diode laser, a solid state laser, adiode pumped solid state (DPSS) laser, and various gas lasers (such as ahelium neon laser, an argon laser, etc.). In some cases, light source 29may be implemented with a high-power, non-laser-based source, such as anarc lamp, mercury high or low-pressure lamp, an LED array, a light bulb,etc. The beam of light generated by light source 29 (i.e., the“generated light”) is directed to a surface of specimen 14 through beamforming and polarization control optics 12. To eliminate confusion, thelight that reaches the surface of the specimen will be referred toherein as the “incident light” or the “incident laser beam.” Asdescribed in more detail below, the “incident light” may differ from the“generated light” in one or more ways, including polarization,intensity, size and shape of the spot, etc.

In addition to the elements shown in FIG. 1, the inspection system shownin FIG. 7 may include means for dynamically altering the power level ofthe incident light supplied to the specimen. For example, laser powerattenuator 26 may be arranged between light source 29 and optics 12 fordynamically altering the power level of the incident laser beam during asurface inspection scan. In general, laser power attenuator 26 may beimplemented with a selectively transmissive optical component, which maybe adapted to transmit a portion of the incident light based on apolarization of the incident light. For example, laser power attenuator26 may include a wave plate (such as a quarter-wave plate) and apolarizing beam splitter, in some embodiments. In this configuration,the wave plate may be used to change the polarization of the incominglight, while the beam splitter functions to transmit one or more selectpolarizations (e.g., linearly polarized light) and reflect all others(e.g., randomly, circularly or elliptically polarized light). Byreflecting portions of the light, the wave plate and beam splitterfunction to reduce the intensity or power level of the light transmittedthere through. However, wave plates and similar optical components (suchas neutral density filters) cannot be turned on and off like a switch,and instead, must be moved in and out of the beam path to provide twodistinct power levels. In some cases, such movement may not be fastenough to provide dynamic power alteration during a surface inspectionscan.

FIG. 8 illustrates one embodiment of a preferred laser power attenuator26. In the embodiment shown, extremely fast laser power attenuation isprovided by using an electro-optical material 200 to switch between an“pass-through” condition and an “attenuate” condition. When “on,” theelectro-optical material 200 may change the polarization of the incominglight into a predetermined polarization orientation. This so-called“re-polarized light” may then be supplied to a polarizing beam splitter210, which may transmit only a portion of the re-polarized light,depending on the particular polarization output from the electro-opticalswitch. Remaining portions of the re-polarized light may be reflectedand absorbed by beam dump 220. In some cases, the electro-opticalmaterial may switch between “pass-through” and “attenuate” conditionswithin a time span of a few nanoseconds to a few microseconds. In thismanner, fast laser power attenuation can be provided by using anelectro-optical switch, rather than moving a selectively transmissiveoptical element in and out of the beam path.

In a specific embodiment, laser power attenuator 26 may include ahigh-speed, bi-refringent electro-optical component known as a Pockel'sCell. Initially, Pockel's Cell 200 may be configured to impart apolarization to incident light whereby the polarization is such that atleast a portion of the resultant light will pass through the polarizingbeam splitter 210. However, when the presence of a large particle isdetected on a specimen 14, the Pockel's Cell 200 may be switched to aconfiguration where the Pockel's Cell 200 changes the polarization ofthe light generated within the light source 29 to a polarization thatcan be at least partially filtered out by polarizing beam splitter 210.In one example, the voltage supplied to Pockel's Cell 200 (i.e., causingthe cell to switch to “attenuate” condition) may alter thecharacteristics of the electro-optical crystal so that it changeslinearly polarized light into circularly polarized light, a phenomenonfrequently referred to as a “quarter wave phase shift.” If thecircularly polarized light is supplied to a beam splitter which isprimarily configured for reflecting circularly polarized light, theintensity or power level of the light output from laser power attenuator26 can be reduced by setting the Pockel's Cell 200 in the “attenuate”condition. On the other hand, the intensity or power level of the lightoutput from laser power attenuator 26 can be maintained (or increased)by setting the Pockel's Cell 200 in a “pass-through” condition.

However, the intensity of the light output from power attenuator 26 isdependent on polarizing beam splitter 210, as well as the phase shiftproduced by Pockel's cell 200. For example, beam splitters typicallydiscriminate between two orthogonal polarizations such as, e.g., theso-called “S” and “P” polarizations. However, other polarizations oflight (such as C-polarized light) may be partially transmitted, andtherefore, partially redirected (e.g. into the beam dump) by the beamsplitter. If a voltage is applied such that the Pockel's cell creates a¼ wave phase shift, incoming linearly polarized light (typical laseroutput) will become circularly polarized and half of that light willpass through the beam splitter, while the other half is redirected. Fora ½ wave shift, no light will pass through the beam splitter except forsome leakage due to imperfection of the optical components. In otherwords, virtually all of the incoming light will be redirected when thePockel's Cell is configured to produce a ½-wave shift (assuming that inthe power off state all light passes through the beam splitter).

Other means may be used for dynamically altering a power level of theincident light supplied to a specimen, in addition to the examplesdescribed above and shown in FIGS. 7 and 8. For example, such means mayinclude, but are not limited to, direct power adjustment of the lightsource, a fast micro mirror, an acousto-optical deflector (AOD), and afast mechanical shutter. As such, the present invention may encompassany appropriate means for dynamically altering the power level of alaser beam, given that such means provide a relatively fast response(e.g., on the order of a few nanoseconds to a few microseconds) and atleast two distinct power levels (e.g., “safe” and “full” power levels).In general, the response time should be faster than the typical time ittakes to damage a particle. Other factors that may influence the choiceof a fast laser power attenuator include, but are not limited to,optical transmission, cost, reliability and life-time.

Referring to FIGS. 13-15, alternate exemplary configurations of aninspection system 10 are shown. As described above, the means fordynamically altering a power level of the incident light supplied to aspecimen 14 may include direct power adjustment of the light source 29.As shown in FIG. 13, the laser power attenuator 26 may be incorporatedas a component of the light source 29 itself in order to dynamicallyalter the power level of the output laser beam of the light source 29during a surface inspection scan. The laser power attenuator 26 may bedisposed in at least one optical path within the light source 29 tomodify the properties of one or more intermediate beams prior to outputof a specimen illumination beam 340 by the light source 29.

As described above, the laser power attenuator 26 may be implementedwith a selectively transmissive optical component, which may be adaptedto transmit a portion of the incident light based on a polarization ofthe incident light. For example, laser power attenuator 26 may includean electro-optical material 200 (such as a ¼-wave plate) and apolarizing beam splitter 210. In this configuration, the electro-opticalmaterial 200 may be used to change the polarization of the incominglight, while the polarizing beam splitter 210 functions to transmit oneor more select polarizations (e.g., linearly polarized light) andreflect all others (e.g., randomly, circularly or elliptically polarizedlight). By reflecting portions of the light, the electro-opticalmaterial 200 and polarizing beam splitter 210 function to reduce theintensity or power level of the light transmitted therethrough.

FIGS. 14 and 15 illustrate additional exemplary embodiments of lightsource 29. In the embodiment shown, extremely fast laser powerattenuation may be provided by using the electro-optical material 200 toswitch between an “pass-through” condition and an “attenuate” condition.When “on,” the electro-optical material 200 may change the polarizationof the incoming light into a predetermined polarization orientation.This so-called “re-polarized light” may then be supplied to a polarizingbeam splitter 210, which may transmit only a portion of the re-polarizedlight, depending on the particular polarization output from theelectro-optical switch. Remaining portions of the re-polarized light maybe reflected and absorbed by beam dump 220. In some cases, theelectro-optical material may switch between “pass-through” and“attenuate” conditions within a time span of a few nanoseconds to a fewmicroseconds. In this manner, fast laser power attenuation can beprovided by using an electro-optical switch, rather than moving aselectively transmissive optical element in and out of the beam path.

In a specific embodiment, laser power attenuator 26 may include ahigh-speed, bi-refringent electro-optical component known as a Pockel'sCell. Initially, Pockel's Cell 200 may be configured to impart apolarization to incident light whereby the polarization is such that atleast a portion of the resultant light will pass through the polarizingbeam splitter 210. However, when the presence of a large particle isdetected on a specimen 14, the Pockel's Cell 200 may be switched to aconfiguration where the Pockel's Cell 200 changes the polarization ofthe light generated within the light source 29 to a polarization thatcan be at least partially filtered out by polarizing beam splitter 210.In one example, the voltage supplied to Pockel's Cell 200 (i.e., causingthe cell to switch to “attenuate” condition) may alter thecharacteristics of the electro-optical crystal so that it changeslinearly polarized light into circularly polarized light, a phenomenonfrequently referred to as a “quarter wave phase shift.” If thecircularly polarized light is supplied to a beam splitter which isprimarily configured for reflecting circularly polarized light, theintensity or power level of the light output from laser power attenuator26 can be reduced by setting the Pockel's Cell 200 in the “attenuate”condition. On the other hand, the intensity or power level of the lightoutput from laser power attenuator 26 can be maintained (or increased)by setting the Pockel's Cell 200 in a “pass-through” condition.

In one example, the voltage supplied to Pockel's Cell 200 (i.e., causingthe cell to switch to the “pass-through” condition) may alter thecharacteristics of the electro-optical crystal so that it changeslinearly polarized light into circularly polarized light, a phenomenonfrequently referred to as a “quarter wave phase shift.” If thecircularly polarized light is supplied to a polarizing beam splitter210, which is primarily configured for reflecting circularly polarizedlight, the intensity or power level of the light output from laser powerattenuator 26 can be reduced by setting the Pockel's Cell 200 in the“pass-through” condition. On the other hand, the intensity or powerlevel of the light output from laser power attenuator 26 can bemaintained (or increased) by setting the Pockel's Cell 200 in the“attenuate” condition.

The intensity of the light output from the laser power attenuator 26 isdependent on polarizing beam splitter 210, as well as the phase shiftproduced by Pockel's Cell 200. For example, beam splitters typicallydiscriminate between two orthogonal polarizations such as, e.g., theso-called “S” and “P” polarizations. However, other polarizations oflight (such as C-polarized light) may be partially transmitted, andtherefore, partially redirected (e.g. into the beam dump 220) by thepolarizing beam splitter 210. If a voltage is applied to the Pockel'sCell 200 by the variable power supply 230 in response to a controlsignal such that the Pockel's Cell 200 creates a ¼-wave phase shift,incoming linearly polarized light (typical laser output) will becomecircularly polarized and half of that light will pass through the beamsplitter, while the other half is redirected. For a ½ wave shift, nolight will pass through the beam splitter except for some leakage due toimperfection of the optical components. In other words, virtually all ofthe incoming light will be redirected when the Pockel's Cell 200 isconfigured to produce a ½ wave shift (assuming that in the power offstate all light passes through the beam splitter). It may be desirableto operate the inspection system 10 in the ranges of deep ultraviolet(DUV) at about 266 nm or ultraviolet (UV) at about 355 nm to accuratelyinspect the surface features. However, disadvantages of the use of suchinspection wavelengths include the long-term durability of DUV coatings,the durability of various electro-optical materials (e.g. Pockel's Cell200 materials) under exposure to UV as well as UV-inducedphotocontamination of the surfaces of a specimen 14. Further, typicalPockel's Cell 200 materials used in UV/DUV-band inspection include BBO.In contrast, IR-band Pockel's Cell 200 materials include KD*P crystals,which can be grown large to large sizes and more efficiently. FIG. 16presents specifications for various Pockel's Cell materials which may beemployed.

Various coating materials may exhibit greater durability under bothvisible and IR radiation. As such, it may be advisable to dispose thelaser power attenuator 26 within the optical path of either the IRcomponent or the green component of the UV laser 20 to avoid directillumination by the UV or DUV band radiation. Further, UV lasers withoutput on the order of 1 Watt in the ultraviolet may have 10's of Wattsof IR inside the cavity, and a similar amount of Green. Therefore, aPockel's Cell inside the laser may indeed experience more incidentpower. However, reliably accommodating large amounts of IR and Visiblepower is a much easier task then accommodating even 1/10^(th) the powerin the UV.

Referring to FIGS. 14 and 15, a UV-band mode-locked UV laser 20 thatincludes the laser power attenuator 26 may be employed. A UV laser 20may generate both infrared (IR) and green components prior to aconversion of the components into a DUV signal. For example, the UVlaser 20 may include three separate stages: an infrared generator 270(e.g. diode laser pump 271 and infrared cavity and amplifier 282), aharmonic stage which converts the IR-band radiation into visible-bandradiation (e.g. visible harmonic generation crystal and optics 290configured to generate green-band radiation), and a second harmonicstage which converts the visible-band radiation into ultraviolet bandradiation (e.g. UV harmonic generation crystal and optics 300 configuredto generate UV-band radiation).

As described above, it may be possible to use Pockel's Cell 200 toquench a laser beam without the use of polarizing optics in the opticalpath of the light source 29. Harmonic conversion crystals (e.g. visibleharmonic generation crystal and optics 290 and UV harmonic generationcrystal and optics 300) typically require a particular polarizationstate of light incident for efficient conversion to a shorterwavelength. If the Pockel's Cell 200 is energized and rotates thepolarization of incident light, the light beam subsequently incident ona harmonic crystal will then not be of the required polarization togenerate the shorter wavelength, thereby attenuating the output beam340. In such a scenario, the The Pockel's Cell 200 may be disposed atdifferent locations in the optical path of the light source 29.

Referring to FIG. 14, the Pockel's Cell 200 may be positioned in theoptical path of the light source 29 between the infrared generator 270and the visible harmonic generation crystal and optics 290. The Pockel'sCell 200 may receive IR-band radiation 310 (e.g. IR wavelengths of about0.7 to 1000 μm) and attenuate that IR-band radiation 310 (as describedabove) to provide attenuated IR-band radiation 320. The attenuatedIR-band radiation 320 may be further converted to visible-band radiation330 (e.g. green-band wavelengths at about 480-550 nm) by the visibleharmonic generation crystal and optics 290 and to attenuated UV-bandradiation 340 by the UV harmonic generation crystal and optics 300 (e.g.DUV at about 266 nm; UV at about 355 nm).

Alternately, referring to FIG. 15, the Pockel's Cell 200 may be disposedbetween the visible harmonic generation crystal and optics 290 and theUV harmonic generation crystal and optics 300. The Pockel's Cell 200 mayreceive visible-band radiation 330 from the visible harmonic generationcrystal and optics 290 and attenuate that visible-band radiation 330 (asdescribed above) to provide attenuated visible-band radiation 350. Theattenuated visible-band radiation 350 may be further converted toattenuated UV-band radiation 340 by the UV harmonic generation crystaland optics 300.

In some cases, the constant power laser beam generated by light source29 can be divided into two distinct power levels (e.g., a “safe” powerlevel and a “full” power level) by dynamically switching anelectro-optical shutter (such as a Pockel's Cell) between “pass-through”and “attenuate” conditions. The safe power level may be substantiallyless than the full power level to prevent thermal damage when scanningover large particles. For example, the safe power level may be somepercentage (ranging, e.g., between about 1% and about 50%) of the fullpower level. In one embodiment, the safe power level may be about 10% ofthe full power level. Other possibilities exist and may generally dependon the incident laser power, as well as the size and materialcomposition of the particles being scanned.

In other cases, an electro-optical shutter (such as a Pockel's Cell) maybe configured for generating more than two distinct power levels. Forexample, a Pockel's Cell can be driven to produce substantially anyphase shift, and thus, may be combined with a polarizing beam splitterto create substantially any output power level. In other words, theembodiment shown in FIG. 8 could be used to create substantially anynumber of distinct power levels. In some cases, circuitry and/orsoftware may be included to provide a continuous power level adjustment,e.g., in the form of a closed feedback loop, as shown in FIG. 11.

In addition to the various means described above for dynamicallyaltering a power level of a laser beam, the inspection system of FIGS. 7and 13 provide means for controlling such alteration. For example, laserpower controller 28 may be coupled between one or more elements of thedetector subsystem (e.g., collector 16, photodetector 18, amplifier 20,ADC 22 and processor 24) and laser power attenuator 26. As described inmore detail below, laser power controller 28 may continuously monitorthe light scattered from specimen 14 and detected by the detectorsubsystem to determine whether the detected scattered light is above orbelow a predetermined threshold level. Based on such determination,laser power controller 28 may instruct laser power attenuator 26 toprovide the incident light to the specimen at either a first power level(e.g., a “full” power level) or a second power level (e.g., a “safe”power level). The laser power controller may also cause the laser powerattenuator to provide, e.g., a third, fourth, or fifth (and so on) powerlevel to the specimen, if more than two power levels are available andcircumstances warrant (or benefit from) such levels.

In general, the predetermined threshold level may be set to reduce orprevent thermal damage that may be caused when incident light directedto the specimen is absorbed and inadequately dissipated by a feature onthe specimen. The predetermined threshold level is typically based onthe incident laser power density, and more specifically, on a powerdensity associated with the onset of thermal damage inflicted on afeature or particle of certain size. For example, the predeterminedthreshold level may be selected from a group of incident laser powerdensities ranging from about 1 kW/cm² to about 1000 kW/cm² to avoiddamaging relatively large particles (e.g., >5 μm). When scanning organicmaterials, the predetermined threshold level may range from about 1kW/cm² to about 100 kW/cm² to avoid damaging large particles withrelatively poor heat dissipation. As shown in FIG. 7, the predeterminedthreshold level may be supplied from processor (or computer system) 24to laser power controller 28. The predetermined threshold level may beselected manually by a user of the system, or automatically by processor24.

If the detected scattered light remains below the predeterminedthreshold level, laser power controller 28 may instruct laser powerattenuator 26 to maintain the power of the incident laser beam at the“full” power level. However, laser power controller 28 may provideinstructions to reduce the power of the incident laser beam to a “safe”power level, if the detected scattered light exceeds the predeterminedthreshold level (indicating, e.g., that a large particle is near). Oncethe incident laser beam scans over the large particle (or other highlyscattering feature), the detected scattered light may fall back belowthe predetermined threshold level, causing laser power controller 28 toinstruct the laser power attenuator to increase the incident laser beamback to “full” power.

In this manner, the inspection system described herein may be uniquelyconfigured for detecting features of relatively small size by directingthe incident light to the specimen at a first power level (e.g., “full”power), while features of relatively larger size may be detected bydirecting the incident light to the specimen at a second power level(e.g., “safe” power). In the current system, the larger features may bedetected, without inflicting thermal damage on those features, bysetting the second power level substantially lower than the first. Ifmore than two power levels are available, laser power controller 28 maycompare the detected scattered light against two or more thresholdlevels, and instruct the laser power attenuator to maintain, reduce orincrease the incident laser beam to an appropriate power level based onsuch comparison.

FIG. 9 illustrates one embodiment of a preferred laser power controller28. In the embodiment shown, laser power controller 28 includes divider240 for normalizing the detector output against the incident laser powerand detector gain. As such, divider 240 may be used to calculate thenormalized scatter power, and thus, may be alternatively referred to asa scatter power calculator. In one example, the scatter power may becomputed by dividing the detector output by the incident laser power anddetector gain, or:

$\begin{matrix}{{ScatterPower} = \frac{DetectorOutput}{({LaserPower})({DetectorGain})}} & {{EQ}.\mspace{14mu} 10}\end{matrix}$

By normalizing the detector output in such a manner, the laser powercontroller causes the laser power attenuator to consistently switch atthe same scatter light level, rather than switching at a given signallevel. In other words, all signals become larger when the detector gainis increased. If the detector output is not normalized when the detectorgain is increased, switching could occur at a smaller particle size thanactually intended. Normalizing the detector output against incidentlaser power and detector gain enables one to consistently switch (e.g.,to a lower power level) once a particle of a given size is detected.

As shown in FIGS. 9-11, a divider 240 of the laser power controller 28may receive the detector output (i.e., the scattered light detected fromthe specimen) as an analog signal from photodetector 18, oralternatively, as a filtered and digitized signal from ADC 22 orprocessor 24. The detector gain (i.e., the current amplificationassociated with the detector) is supplied to divider 240 by processor24, and may be variable or fixed depending on the particularphotodetector used. As described in more detail below, however, theincident laser power may be supplied to divider 240 in one of two ways.

In some embodiments, the normalized scatter power signal generated bydivider 240 may be fed back to processor 24, as shown by the dottedlines in FIGS. 7 and 9-11. In other words, divider 240 may be used topresent the scatter data (i.e., the detector output) as a scan result,which has been normalized against incident laser power and detectorgain. By normalizing the data before it is sent to the processor, theactual defect scatter power can be used to accurately detect the size ofa defect. For example, if the incident laser power is lowered whenscanning over a large particle, the ADC counts (i.e., the detectoroutput) will necessarily be lower than in the unattenuated case. Thismeans that the scan results supplied to the processor will show asmaller defect than what is actually there. Normalizing the scatter dataenables the processor to more accurately determine the size of thedefect.

In some embodiments, an additional normalizer/divider 23 may be used inthe data collection path between ADC 22 and processor 24 for normalizingthe scatter power signal against changes in incident laser power. Theadditional divider 23 may be used along with, or as an alternative to,the normalizer/divider (240) included within laser power controller 28.For example, if two dividers are used, divider 23 may be placed in thedata collection path for normalizing the scatter power signal sent toprocessor 24, while divider 240 is placed in the threshold path fornormalizing the scatter power signal sent to another laser powercontroller component (e.g., comparator 250, as discussed below).However, there may exist other options in which: only one divider isused (either in the data collection path or the threshold path), or nodividers are used (in which case, the system would not support dynamicrange extension, as discussed below).

As noted above, the incident laser power may be supplied to divider 240in one of two ways. In the embodiment of FIG. 9, the actual laser powerof the incident beam is measured by laser power detector 27, which maybe arranged in the beam path below laser power attenuator 26. In thisembodiment, laser power detector 27 is included to monitor the actualintensity or power level output from laser power attenuator 26. Thepower measured by the laser power detector (i.e., the Measured Power) issupplied to laser power controller 28 for calculating the normalizedscatter power. The laser power detector may be implemented withsubstantially any power detecting means including, but not limited to, aphotodiode and a photomultiplier tube (PMT), among others. As describedin more detail below, however, laser power detector 27 may not beincluded in all embodiments of the invention.

As shown in FIG. 9, laser power controller 28 may also include acomparator 250 for comparing the normalized scatter power to one or morepredetermined threshold levels supplied by processor 24. As noted above,the threshold level(s) may be selected by a user or processing componentof the system to effectively reduce thermal damage when scanning overlarge (or other highly scattering) particles. In one embodiment,comparator 250 may receive only one threshold level (referred to as a“safe scatter threshold”), which indicates a laser power densityassociated with a maximum amount of “safe” scatter power. Aftercomparing the normalized scatter power to the safe scatter threshold,comparator 250 may instruct laser power attenuator 26 to maintain,reduce or increase the incident laser beam to an appropriate power level(e.g., a “full” or “safe” power level) by maintaining or changing thePower Mode supplied to the attenuator. In general, the Power Mode may beany control signal that causes the attenuator to maintain or change theoutput power level. In the embodiment of FIG. 8, for example, the PowerMode supplied to the attenuator may be functionally equivalent to thecontrol signal input to variable power supply 230.

FIG. 10 illustrates another embodiment of a preferred laser powercontroller 28. In particular, FIG. 10 illustrates one manner in whichthe normalized scatter power can be calculated if a laser power detectoris not used to provide a measurement of the actual power level outputfrom laser power attenuator 26. Instead of receiving the Measured Power,divider 240 may be coupled to the output of comparator 250 for receivingthe Power Mode control signal supplied to the attenuator. Based on thecontrol signal, divider 240 may determine the appropriate incident laserpower to be used in the scatter power calculations by means of, e.g., alook up table. In the embodiments shown in FIGS. 9 and 10, divider 240and comparator 250 may be implemented with hardware, software or acombination of both. In one example, divider 240 may be implemented insoftware, whereas comparator 250 may be implemented in hardware.

FIG. 11 illustrates yet another embodiment of a preferred laser powercontroller 28. In particular, FIG. 11 illustrates one manner in whichcontinuous power adjustment may be provided by laser power controller28. Like the previous embodiment, divider 240 may be coupled forreceiving the detector gain and output signals, and for generating anormalized scatter power signal in response thereto. However, instead ofsupplying the normalized scatter power signal to a comparator (as shownin FIGS. 9-10), the scatter power signal is supplied to a control loopfeedback filter 260, which dynamically adjusts the output Power Modebased on the supplied signal. In FIG. 11, the scatter power signal isused in the feedback loop 260 to adjust the incident laser power, e.g.to achieve a constant detector output signal. Therefore, instead offixed power levels, the embodiment shown in FIG. 11 provides acontinuously adjustable power level.

The circuits and systems shown in FIGS. 7-11 may reduce thermal damageto large particles (e.g., >5 μm) by dynamically adjusting the intensityor power level of the incident laser beam during a surface inspectionscan. In one example, thermal damage may be reduced by as much as 100%over fixed incident laser beam inspection systems. The circuits andsystems described herein may be tailored to a variety of scan operationsby providing one or more preset threshold levels, which may be used fordynamically switching between two or more incident beam power levelsduring the scan operation. In this manner, thermal damage may bereduced, or even avoided, by reducing the incident laser power to alower power level (e.g., a “safe” power level) when scanning over large,highly scattering particles. However, detection sensitivity ismaintained by scanning lower-scatter regions at a higher power level(e.g., “full” power level) which allows the system to detect smallerdefects.

In alternative embodiments of the invention, an adaptive learningprocess may be used for altering the threshold levels and/or powerlevels based on previous or current inspection scan results. As oneadvantage, an adaptive learning process would allow a longer delay timefor the switching electronics, because the decision would be made far inadvance of the actual switching event, rather than “on the fly” justbefore switching is needed. Though accuracy may be increased, such anembodiment would obviously increase the complexity (and probably thecost) of the circuits and systems described above. As a mid-rangealternative, a scaled relationship between scatter power and incidentpower levels may be established, in some embodiments, for continuouslyaltering the amount of incident power supplied to the specimen. Thisalternative could be used to provide a scatter light signal that isalways near the optimal range for the PMT, due to the continualadjustment of the incident power supplied to the specimen.

Though the use of a single photodetector is preferred in mostembodiments of the invention, an additional detector may be included insome embodiments for selecting an appropriate power level to be suppliedto the specimen. If included, the separate detector may be used tomonitor the light scattered from the specimen. However, unlike theoriginal detector, which is used for detecting the scattered light sothat the incident power level may be adjusted accordingly, theadditional detector may be used for selecting a particular thresholdlevel or “switch point”, which may then be used for selecting anappropriate power level to be directed to the specimen.

In addition to reducing thermal damage, the circuits and systems shownin FIGS. 7-11 may also be used to increase the measurement detectionrange of an inspection system. Usually, the detection range of afixed-power inspection system is limited to the detection range of thephotodetector. However, by providing a variable-power inspection system,the present invention significantly increases the measurement detectionrange by approximately:(Photodetector detection range)×(Attenuator detection range)  EQ. 11

In some cases, the additional detection range provided by the laserpower attenuator may increase the overall measurement detection range ofthe system by about 2 times to 16 times. When combined with other meansdescribed herein (such as the improved PMT detector and/or dual-outputamplifier), the overall measurement detection range of the inspectionsystem may be improved by about 2 times to 10,000 times overconventional techniques.

FIG. 12 is a flow chart diagram illustrating an exemplary method forinspecting a specimen with a variable power inspection system, such asthe one shown in FIGS. 7-11 and 13-15 and described above. Variousmethod steps set forth in FIG. 12 may be performed by componentsincluded within the inspection system, although certain steps may beperformed by a user of the inspection system.

In some embodiments, the method may begin by directing light to aspecimen at a first power level (in step 300). For example, an incidentlaser beam may be supplied to the specimen at a relatively high powerlevel (such as a “full” power level) so that relatively small featuresor defects can be detected. As described in more detail below, theincident laser beam may be subsequently reduced to a lower power level(such as a “safe” power level) so that relatively larger features ordefects can be detected without damaging those features. The methodshown in FIG. 12 can be modified for dynamically switching between morethan two power levels, as desired.

In most cases, the method may detect light scattered from the specimen(in step 304) while scanning the light over a surface of the specimen(in step 302). For example, scattered light may be detected from thespecimen when an incident laser beam is directed to a current locationon the specimen. The scattered light detected at the current locationmay be used (in step 306) for detecting features, defects or lightscattering properties of the specimen at that location. The beamposition may then be scanned to a nearby location, where the process isrepeated for detecting features that may be found at the nearbylocation.

In some embodiments, scanning may be implemented by placing an opticaldeflector in the beam path leading to the specimen. For example, thedeflector may be included within beam forming and polarization controloptics 12 of FIGS. 1, 7 and 13. In some embodiments, the deflector mayinclude an acousto-optical deflector (AOD), a mechanical scanningassembly, an electronic scanner, a rotating mirror, a polygon basedscanner, a resonant scanner, a piezoelectric scanner, a galvo mirror, ora galvanometer. In some embodiments, the deflector may scan the lightbeam over the specimen at an approximately constant scanning speed. Forexample, the light beam may be scanned over the specimen at a constantscanning speed selected from a range of speeds between about 0 m/s andabout 24 m/s. In other embodiments, the deflector may scan the lightbeam over the specimen at a variable scanning speed ranging betweenabout 0 m/s to about 24 m/s. However, a deflector may not be needed toimplement scanning in all embodiments of the invention. For example, anormal incidence beam of light may be scanned over the specimen byrelative motion of the beam forming optics with respect to the specimen,and/or by relative motion of the specimen with respect to the optics.

During the scanning process, the method monitors light scattered fromthe specimen at a nearby location (in step 308), while light scatteredfrom the specimen is detected at the current location (in step 304). Forexample, an incident laser beam may be supplied to the specimen with apower density distribution that peaks near the middle of thedistribution and tapers off near the edges of the distribution. As usedherein, the middle of the distribution will be referred to as the “mainbeam,” while the edges are referred to as the “beam skirt.” One examplewould be a power density distribution with a main lobe and at least oneside lobe on each side of the main lobe; the side lobes having a smallerpower density, and therefore, a smaller amplitude than the main lobe.Another example is a bell-shaped or Gaussian distribution whose one mainlobe has a gradually tapered beam skirt.

When scanning over the surface of the specimen, the beam skirt of theincident laser beam may reach a particle or defect (e.g., on the orderof one to several microseconds) before the particle or defect is reachedby the main beam. For example, most particles or defects on the surfaceof a specimen will be significantly smaller than the spot size of thelaser beam. This enables the beam skirt to reach a particle or defectbefore it is reached by the main beam. As the incident laser beam isscanned over the surface of the specimen, the amount of light scatteredfrom the specimen (i.e., the scatter power) will change depending onwhat part of the beam is covering the particle or defect. Assume, forexample, that the high power density main beam covers a relativelysmooth surface of the specimen when the low power density beam skirtreaches a large particle or defect. Because the amount of lightscattered from relatively smooth surfaces is typically a lot smallerthan the scatter attributed to large particles or defects, the amount ofscatter power attributed to the main beam portion may be considerednegligible. As such, a significant increase in the scatter power mayindicate that the beam skirt has reached a large particle or defect. Inother words, the presence of large particles or defects may be detectedat a nearby location by monitoring the scatter levels from the low powerdensity beam skirt.

If the scatter levels from the beam skirt are substantially greater thanor equal to the safe scatter threshold (in step 310), the incident laserbeam may be reduced to a second power level, lower than the first (instep 314). As noted above, the second power level (referred to as the“safe” power level) may typically range between about 1% to about 50% ofthe first power level (referred to as the “full” power level), and in apreferred embodiment, may be about 10% of the full power level. If thescatter levels from the beam skirt are less than the safe scatterthreshold (in step 310), the first power level will be maintained toenable smaller features to be detected and to preserve detectionsensitivity.

In some cases, the method may end (in step 318), if the surfaceinspection scan is complete (in step 316). Otherwise, the scanningprocess may continue (in step 320) so that the nearby location becomesthe current location. The method continues to monitor the beam skirtscatter levels while the incident laser beam moves over the defect. Whenthe scatter levels fall back below the safe scatter threshold (in step310), indicating that the high-density center of the beam (i.e., themain beam) has passed the defect, the first power level will be restored(in step 312) to continue inspecting the specimen for small defects.

By using a fast laser power attenuator, such as those described aboveand shown in FIGS. 7 and 8, the systems and methods described herein mayeasily switch between high and low power levels before the main beamreaches the nearby location. For example, beam skirt scatter levels maybe used to indicate the presence of a large defect at a nearby locationseveral microseconds before the main beam reaches that location. Due inpart to the relatively fast response (e.g., on the order of a fewnanoseconds to a few microseconds) of laser power attenuator 26, thepresent invention is able to reduce the incident beam power level beforethe main beam reaches the defect by monitoring the beam skirt scatterlevels. By dynamically decreasing the power level while scanning largeparticles, and increasing the power level once the particle is scanned,the present invention reduces thermal damage to large particles, whilemaintaining system throughput and sensitivity.

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. For example, methods and systems for extending thedetection range of an inspection system are provided. Accordingly, thisdescription is to be construed as illustrative only and is for thepurpose of teaching those skilled in the art the general manner ofcarrying out the invention. It is to be understood that the forms of theinvention shown and described herein are to be taken as the presentlypreferred embodiments. Elements and materials may be substituted forthose illustrated and described herein, parts and processes may bereversed, and certain features of the invention may be utilizedindependently, all as would be apparent to one skilled in the art afterhaving the benefit of this description of the invention. Changes may bemade in the elements described herein without departing from the spiritand scope of the invention as described in the following claims.

1. An inspection system comprising: an illumination subsystem fordirecting light to an inspection specimen comprising: a radiationsource; a radiation wavelength converter; a power attenuator subsystemconfigured for altering the power level of a light beam emitted by theradiation source of the illumination subsystem, wherein the powerattenuator subsystem is disposed in an optical path between theradiation source and the radiation wavelength converter; and a powerattenuation control subsystem configured to provide control signals tothe power attenuator subsystem according to a detected level of lightscattering by the inspection specimen upon illumination by theillumination subsystem.
 2. The inspection system of claim 1, wherein theradiation source comprises: an infrared radiation source.
 3. Theinspection system of claim 1, wherein the radiation wavelength convertercomprises: a visible-band radiation wavelength converter.
 4. Theinspection system of claim 3, wherein the visible-band radiationwavelength converter comprises: a green-band radiation wavelengthconverter.
 5. The inspection system of claim 1, wherein the powerattenuation subsystem is configured for: substantially maintaining apower of the light beam emitted by the radiation source of theillumination subsystem where the level of light scattering remains witha threshold range; reducing the power of the light beam emitted by theradiation source of the illumination subsystem if the detected level oflight scattering exceeds the threshold range; and increasing the powerof the light beam emitted by the radiation source of the illuminationsubsystem if the detected level of light falls below the thresholdrange.
 6. The inspection system of claim 5, wherein the threshold rangeis selected according to an identity of the inspection specimen.
 7. Theinspection system of claim 1, wherein the power attenuation subsystemcomprises: a Pockel's cell.
 8. A method for scatterometry inspectioncomprising: generating light of a first wavelength utilizing a radiationsource of a light source; converting the light of the first wavelengthto light of a second wavelength utilizing a radiation wavelengthconverter; directing a portion of the light of the second wavelengthhaving a power level to an inspection specimen; detecting light of thesecond wavelength scattered from the inspection specimen; and modifyinga power level of one or more light beams of the first wavelengthemanating from the radiation source of the light source according to alevel of light scattering by the specimen upon illumination by the lightsource.
 9. The method of claim 8, wherein the modifying a power level ofone or more light beams of the first wavelength emanating from theradiation source of the light source according to a level of lightscattering by the specimen upon illumination by the light sourcecomprises: upon detection of light scattering within a threshold range,substantially maintaining the power level of one or more light beams ofthe first wavelength emanating from the radiation source of the lightsource; upon detection of light scattering exceeding the thresholdrange, reducing the power level of one or more light beams of the firstwavelength emanating from the radiation source of the light source; andupon detection of light scattering below the threshold range, increasingthe power level of one or more light beams of the first wavelengthemanating from the radiation source of the light source.
 10. Aninspection system comprising: an illumination subsystem for directinglight to an inspection specimen comprising: a radiation source; a firstradiation wavelength converter; and a second radiation wavelengthconverter; a power attenuator subsystem configured for altering thepower level of a light beam emitted by the first radiation wavelengthconverter of the illumination subsystem, wherein the power attenuatorsubsystem is disposed in an optical path between the first radiationwavelength converter and the second radiation wavelength converter, anda power attenuation control subsystem configured to provide controlsignals to the power attenuator subsystem according to a detected levelof light scattering by the inspection specimen upon illumination by theillumination subsystem.
 11. The inspection system of claim 10, whereinthe radiation source comprises: an infrared radiation source.
 12. Theinspection system of claim 10, wherein the first radiation wavelengthconverter comprises: a visible-band radiation wavelength converter. 13.The inspection system of claim 12, wherein the visible-band radiationwavelength converter comprises: a green-band radiation wavelengthconverter.
 14. The inspection system of claim 10, wherein the secondradiation wavelength converter comprises: a UV-band radiation wavelengthconverter.
 15. The inspection system of claim 10, wherein the powerattenuation subsystem is configured for: substantially maintaining apower of the light beam emitted by the first radiation wavelengthconverter of the illumination subsystem where the level of lightscattering remains with a threshold range; reducing the power of thelight beam emitted by the first radiation wavelength converter of theillumination subsystem if the detected level of light scattering exceedsthe threshold range; and increasing the power of the light beam emittedby the first radiation wavelength converter of the illuminationsubsystem if the detected level of light falls below the thresholdrange.
 16. The inspection system of claim 15, wherein the thresholdrange is selected according to an identity of the inspection specimen.17. The inspection system of claim 10, wherein the power attenuationsubsystem comprises: a Pockel's cell.
 18. A method for scatterometryinspection comprising: generating light of a first wavelength utilizinga radiation source of a light source; converting light of the firstwavelength to light of a second wavelength utilizing a first radiationwavelength converter; converting light of the second wavelength to lightof a third wavelength utilizing a second radiation wavelength converter;directing a portion of light of the third wavelength having a powerlevel to an inspection specimen; detecting light of the third wavelengthscattered from the inspection specimen; and modifying a power level ofone or more light beams of the second wavelength emanating from thefirst wavelength converter according to a level of light scattering bythe specimen upon illumination by the light source.
 19. The method ofclaim 18, wherein the modifying a power level of one or more light beamsof the second wavelength emanating from the first wavelength converteraccording to a level of light scattering by the specimen uponillumination by the light source comprises: upon detection of lightscattering within a threshold range, substantially maintaining the powerlevel of one or more light beams of the second wavelength emanating fromthe first wavelength converter; upon detection of light scatteringexceeding the threshold range, reducing the power level of one or morelight beams of the second wavelength emanating from the first wavelengthconverter; and upon detection of light scattering below the thresholdrange, increasing the power level of one or more light beams of thesecond wavelength emanating from the first wavelength converter.